fix crash in ecp_nistz256_point_add_affine()

The .rodata section with precomputed constant `ecp_nistz256_precomputed` needs to be
terminated by .previous. The lack of .previous makes mingw compiler to put code
into read only section. The exception is raised as soon as CPU attempts to execute
the code from read only section.

Fixes #24184

crypto/ec/asm/ecp_nistz256-x86_64.pl

@@ -4736,6 +4736,7 @@ ()
 }
 print <<___;
 .size	ecp_nistz256_precomputed,.-ecp_nistz256_precomputed
+.previous
 ___
 
 $code =~ s/\`([^\`]*)\`/eval $1/gem;